Harmonic analysis of hydrogeological time-series: a new high-resolution method to characterize highly heterogeneous coastal aquifers
Monday, October 5, 2015
Anli Bourhane, Water Office Reunion, Water Resources, Saint Denis, France, Emmanuel Cordier, University of La Reunion, Observatoire des Sciences de l'Univers, Saint Denis, France and Jean-Lambert Join, University of La Réunion, Laboratory Geosciences Reunion, Saint-Denis, France
Abstract:
In recent volcanic islands, the hydrodynamic characterization of coastal aquifers is a real challenge. The interpretation of classic pumping tests is very complicated by 1) a low drawdown related to high permeabilities and 2) the marine influence that can allow a rise of the hydraulic head during the pumping. In this study, hydrogeological time-series were recorded helping a groundwater monitoring network including 12 coastal boreholes situated between 300 and 3000 m from the shoreline, in the coastal aquifer of Pierrefonds (in Reunion Island). The geological setting is characterized by fissured basalt layers covered by a thick alluvial deposit that can reach 50 m of thickness. Several pyroclastic levels and lahars or mudslides products were intercepted by the existing boreholes. This aquifer presents a high degree of spatial heterogeneity in relation with 1) the diversity of geological formations and 2) the variable degree of confinement due to variable weathering patterns and superficial cover. The analysis of the attenuation of the tidal signal by the aquifer matrix enables to derive its hydraulic diffusivity D. Using a signal analysis script (T-Tide) written on Matlab, theoretical tidal constituents were identified in all the collected groundwater head time-series. Diffusivity values calculated for significant constituents have allowed to identify the confined part of the aquifer. The analysis of the amplitude of the signal-to-noise ratio (SNR) on the various tidal components in each borehole shows that the confined groundwater body is more sensitive to the high-frequency tidal component (M2), whereas unconfined groundwater bodies are characterized by a sensitivity to the low-frequency tidal component (SA). These results have highlighted the efficiency of time-series harmonic analysis for a high-resolution hydrodynamic characterization in these complex coastal environments.
